U.S. patent number 10,660,773 [Application Number 15/432,837] was granted by the patent office on 2020-05-26 for crimping methods for thin-walled scaffolds.
This patent grant is currently assigned to ABBOTT CARDIOVASCULAR SYSTEMS INC.. The grantee listed for this patent is Abbott Cardiovascular Systems Inc.. Invention is credited to Edward P. Garcia, Boyd V. Knott, Jill A. McCoy, Ashleigh Z. Sheehy, Karen J. Wang.
United States Patent |
10,660,773 |
Wang , et al. |
May 26, 2020 |
Crimping methods for thin-walled scaffolds
Abstract
A medical device includes a balloon expanded scaffold crimped to
a catheter having a balloon. The scaffold has a network of rings
formed by struts connected at crowns and links connecting adjacent
rings. The scaffold is crimped to the balloon by a process that
includes using protective polymer sheaths or sheets during
crimping, and adjusting the sheaths or sheets during the crimping
to avoid or minimize interference between the polymer material and
scaffold struts as the scaffold is reduced in size.
Inventors: |
Wang; Karen J. (Sunnyvale,
CA), Garcia; Edward P. (Dublin, CA), Knott; Boyd V.
(Menifee, CA), McCoy; Jill A. (Sunnyvale, CA), Sheehy;
Ashleigh Z. (Santa Clara, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Abbott Cardiovascular Systems Inc. |
Santa Clara |
CA |
US |
|
|
Assignee: |
ABBOTT CARDIOVASCULAR SYSTEMS
INC. (Santa Clara, CA)
|
Family
ID: |
61913522 |
Appl.
No.: |
15/432,837 |
Filed: |
February 14, 2017 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20180228630 A1 |
Aug 16, 2018 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B29C
66/63 (20130101); B29C 66/71 (20130101); B29C
66/532 (20130101); B29C 65/7817 (20130101); B29C
66/73791 (20130101); B29C 66/73117 (20130101); B29C
67/0014 (20130101); B29C 65/66 (20130101); A61F
2/958 (20130101); B29C 66/71 (20130101); B29K
2067/046 (20130101); B29C 66/91945 (20130101); B29C
2793/0009 (20130101); A61F 2/9522 (20200501); A61F
2002/91566 (20130101); B29L 2031/7543 (20130101); B29C
66/91411 (20130101); A61F 2/04 (20130101); B29C
66/949 (20130101); B29C 65/8246 (20130101); B29L
2028/00 (20130101); B29C 66/929 (20130101); A61F
2/915 (20130101); B29C 66/91941 (20130101); B29C
66/71 (20130101); B29K 2067/043 (20130101) |
Current International
Class: |
B29C
65/66 (20060101); B29C 65/78 (20060101); A61F
2/958 (20130101); B29C 65/00 (20060101); B29C
67/00 (20170101); A61F 2/95 (20130101); A61F
2/915 (20130101); B29C 65/82 (20060101); A61F
2/04 (20130101) |
References Cited
[Referenced By]
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Other References
US. Appl. No. 11/330,927, filed Jan. 11, 2006, Wu et al. cited by
applicant .
U.S. Appl. No. 11/938,127, filed Nov. 9, 2007, Wang. cited by
applicant .
Angioplasty Summit Abstracts/Oral, Am J Cardiol. Apr. 23-26, 2013,
p. 23B. cited by applicant .
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AbsorbTM bioresorbable vascular scaffold", Interv Cardiol. 2012;
4(6): 621-631. cited by applicant .
Miller, R., "Abbott's Bioresorbable Stent Shows Durable Results in
ABSORB Trial", The Gray Sheet, Mar. 25, 2013, pp. 17-18. cited by
applicant .
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factor-incorporated degradable stent: comparison with traditional
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pages. cited by applicant.
|
Primary Examiner: Afzali; Sarang
Assistant Examiner: Hidalgo-Hernandez; Ruth G
Attorney, Agent or Firm: Squire Patton Boggs (US) LLP
Claims
What is claimed is:
1. A method, comprising: using a scaffold made from a tube
comprising a polymer, the polymer having a glass transition
temperature, the scaffold having an outer diameter and the outer
diameter having a before crimping size; using a balloon having a
nominal diameter; using a crimping device having a plurality of
blades configured to form an aperture, wherein the blades are
rotated relative to each other to increase or decrease the size of
the aperture during crimping; using a polymer material disposed
within the aperture; and crimping the scaffold to the balloon, the
crimping comprising: placing the scaffold and the balloon within
the aperture, wherein the polymer material is between a surface of
the scaffold and a surface of the blades, reducing the diameter of
the scaffold from the before crimping size to a first size, after
the scaffold has about the first size, resetting the polymer
material within the aperture, reducing the diameter of the scaffold
from the first size to a second size, after the scaffold has about
the second size, resetting the polymer material within the
aperture, and reducing the diameter of the scaffold from the second
size to a third size or a final crimp size.
2. The method of claim 1, wherein the crimping device is a
film-headed crimper.
3. The method of claim 1, wherein the polymer material are polymer
sheets.
4. The method of claim 1, wherein the polymer material comprises a
plurality of sheaths.
5. The method of claim 1, wherein the scaffold has a crimping
temperature during crimping.
6. The method of claim 1, wherein the before crimping size is
greater than the nominal diameter of the balloon.
7. The method of claim 1, wherein the balloon is pressurized during
each of the reducing the diameter steps.
8. The method of claim 1, wherein the crimping step further
includes the step of removing the scaffold and balloon from the
crimping device after the scaffold diameter is reduced to the first
size, then returning the scaffold to the crimping device.
9. The method of claim 8, wherein the resetting of the polymer
material after the scaffold has about the first size occurs when
the scaffold and balloon are removed from the crimping device.
10. The method of claim 8, wherein the balloon is a first balloon,
further including the step of replacing the first balloon with a
second balloon of a balloon catheter when the scaffold is removed
from the crimping device, and the scaffold is crimped to the second
balloon.
11. The method of claim 1, wherein the polymer material within the
aperture is re-set more than 2 times during the crimping.
12. The method of claim 1, wherein before and after reducing the
scaffold diameter from the first size to the second size the
aperture is held constant while the balloon has the nominal
diameter.
13. A method of crimping a stent, comprising: performing a crimping
procedure, the crimping procedure comprising (i) position the stent
over a balloon and within an aperture of a crimping device; (ii)
providing a sheet within the aperture to protect the stent; and
(iii) reducing a diameter of the stent by reducing a diameter of
the aperture of the crimping device; wherein the reducing the
diameter of the stent is performed in multiple stages of diameter
reduction until a final crimped diameter is reached, wherein the
sheet is reset at least 2 times but no more than 5 times until the
stent is crimped to the final crimped diameter, the reset occurring
only when the diameter of the stent is reduced by about 30-35%
between any of the stages, and the reset occurring only when the
aperture of the crimping device is in an open position so as to not
interfere with the resetting of the sheet.
14. The method of claim 13, additionally comprising performing a
pre-crimping procedure, the pre-crimping procedure comprising (a)
placing the stent over the balloon or a pre-crimping balloon; (b)
inflating the balloon or the pre-crimping balloon by application of
a pressure; and (c) reducing the diameter of the stent to a
diameter selected for the start of the crimping procedure, wherein
if a pre-crimping balloon is used, the stent is then transferred to
the balloon from the pre-crimping balloon before the start of the
crimping procedure.
15. The method of claim 13, additionally comprising performing a
pre-crimping procedure, the pre-crimping procedure comprising (a)
placing the stent over the balloon or a pre-crimping balloon; (b)
placing the stent within the aperture of the crimping device; (c)
setting the diameter of the aperture of the crimping device to a
selected size; (d) optionally heating the stent while the diameter
of the aperture is maintained the selected size; (e) inflating the
balloon or the pre-crimping balloon by application of a pressure;
and (f) reducing the diameter of the stent to a diameter selected
for the start of the crimping procedure, wherein if a pre-crimping
balloon is used, the stent is then transferred to the balloon from
the pre-crimping balloon before the start of the crimping
procedure.
16. The method of claim 13, wherein each stage of the multiple
stages of diameter reduction comprises: (a) having the aperture of
the crimping device at a selected size; (b) optionally heating the
stent while the aperture is maintained at the selected size; (c)
inflating the balloon to a selected size by application of a
pressure; (d) reducing the diameter of the stent to a selected size
by reducing the size of the aperture; and (e) maintaining the
aperture of the crimping device for a dwell period of time when the
stent reaches the selected size.
17. A method, comprising: using a scaffold made from a tube
comprising a polymer, the polymer having a glass transition
temperature, the scaffold having an outer diameter and the outer
diameter having a before crimping size; using a balloon having a
nominal diameter; using a crimping device having a plurality of
blades configured to form an aperture; using a polymer material
disposable within the aperture; and crimping the scaffold to the
balloon, the crimping comprising: placing the scaffold and balloon
within the aperture so that the polymer material is between a
scaffold surface and a surface of the blades, reducing the diameter
of the scaffold from the before crimping size to a final crimped
size, wherein the polymer material within the aperture is reset,
when the aperture is in an open state, between 2 and 5 times while
the scaffold diameter is reduced from the before crimping size to
the final crimped size.
18. The method of claim 17, wherein the polymer material comprises
sheaths having different sizes.
19. The method of claim 17, wherein the polymer material are sheets
operated by a film-headed crimper.
20. A method, comprising: using a scaffold made from a tube
comprising a polymer, the polymer having a glass transition
temperature, the scaffold having an outer diameter and the outer
diameter having a before crimping size; using a balloon having a
nominal diameter; using a crimping device having a plurality of
blades configured to form an aperture, wherein the blades are
rotated relative to each other to increase or decrease a size of
the aperture during crimping; using a polymer material disposable
within the aperture; and crimping the scaffold to the balloon, the
crimping comprising: placing the scaffold and balloon within the
aperture, reducing the diameter of the scaffold from the before
crimping size to a first size that is between 30% to 35% less than
the before crimping size, after reducing the diameter to the first
size, increasing the aperture size to remove a pressure of the
blades from a surface of the scaffold, followed by removing excess
polymer material from the aperture, after removing the polymer
material, decreasing the aperture size, reducing the scaffold
diameter from the first size to a second size, after reducing the
diameter to the second size, increasing the aperture size to remove
a pressure of the blades from the surface of the scaffold, followed
by removing excess polymer material from the aperture, after
removing the polymer material, decreasing the aperture size, and
reducing the scaffold diameter from the second size to a third size
or a final crimp size.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to medical devices; more
particularly, this invention relates to processes for uniformly
crimping and deploying a medical device, such as a polymeric
scaffold, to and from, respectively, a delivery balloon.
Description of the State of the Art
Radially expandable endoprostheses are artificial devices adapted
to be implanted in an anatomical lumen. An "anatomical lumen"
refers to a cavity, or duct, of a tubular organ such as a blood
vessel, urinary tract, and bile duct. Stents are examples of
endoprostheses that are generally cylindrical in shape and function
to hold open and sometimes expand a segment of an anatomical lumen.
Stents are often used in the treatment of atherosclerotic stenosis
in blood vessels. "Stenosis" refers to a narrowing or constriction
of the diameter of a bodily passage or orifice. In such treatments,
stents reinforce the walls of the blood vessel and prevent
restenosis following angioplasty in the vascular system.
"Restenosis" refers to the reoccurrence of stenosis in a blood
vessel or heart valve after it has been treated (as by balloon
angioplasty, stenting, or valvuloplasty) with apparent success.
The treatment of a diseased site or lesion with a stent involves
both delivery and deployment of the stent. "Delivery" refers to
introducing and transporting the stent through an anatomical lumen
to a desired treatment site, such as a lesion. "Deployment"
corresponds to expansion of the stent within the lumen at the
treatment region. Delivery and deployment of a stent are
accomplished by positioning the stent about one end of a catheter,
inserting the end of the catheter through the skin into the
anatomical lumen, advancing the catheter in the anatomical lumen to
a desired treatment location, expanding the stent at the treatment
location, and removing the catheter from the lumen.
The stent must be able to satisfy a number of basic, functional
requirements. The stent (or scaffold) must be capable of sustaining
radial compressive forces as it supports walls of a vessel.
Therefore, a stent must possess adequate radial strength. After
deployment, the stent must adequately maintain its size and shape
throughout its service life despite the various forces that may
come to bear on it. In particular, the stent must adequately
maintain a vessel at a prescribed diameter for a desired treatment
time despite these forces. The treatment time may correspond to the
time required for the vessel walls to remodel, after which the
stent is no longer needed.
Scaffolds may be made from a biodegradable, bioabsorbable,
bioresorbable, or bioerodable polymer. The terms biodegradable,
bioabsorbable, bioresorbable, biosoluble or bioerodable refer to
the property of a material or stent to degrade, absorb, resorb, or
erode away from an implant site. Scaffolds may also be constructed
of bioerodible metals and alloys. The scaffold, as opposed to a
durable metal stent, is intended to remain in the body for only a
limited period of time. In many treatment applications, the
presence of a stent in a body may be necessary for a limited period
of time until its intended function of, for example, maintaining
vascular patency and/or drug delivery is accomplished. Moreover, it
has been shown that biodegradable scaffolds allow for improved
healing of the anatomical lumen as compared to metal stents, which
may lead to a reduced incidence of late stage thrombosis. In these
cases, there is a desire to treat a vessel using a polymer
scaffold, in particular a bioabsorbable or bioresorbable polymer
scaffold, as opposed to a metal stent, so that the prosthesis's
presence in the vessel is temporary.
Polymeric materials considered for use as a polymeric scaffold,
e.g. poly(L-lactide) ("PLLA"), poly(D,L-lactide-co-glycolide)
("PLGA"), poly(D-lactide-co-glycolide) or
poly(L-lactide-co-D-lactide) ("PLLA-co-PDLA") with less than 10%
D-lactide, poly(L-lactide-co-caprolactone), poly(caprolactone),
PLLA/PDLA stereo complex, and blends of the aforementioned polymers
may be described, through comparison with a metallic material used
to form a stent, in some of the following ways. Polymeric materials
typically possess a lower strength to volume ratio compared to
metals, which means more material is needed to provide an
equivalent mechanical property. Therefore, struts must be made
thicker and wider to have the required strength for a stent to
support lumen walls at a desired radius. The scaffold made from
such polymers also tends to be brittle or have limited fracture
toughness. The anisotropic and rate-dependent inelastic properties
(i.e., strength/stiffness of the material varies depending upon the
rate at which the material is deformed, in addition to the
temperature, degree of hydration, thermal history) inherent in the
material, only compound this complexity in working with a polymer,
particularly, bioresorbable polymers such as PLLA or PLGA.
Scaffolds and stents traditionally fall into two general
categories--balloon expanded and self-expanding. The later type
expands (at least partially) to a deployed or expanded state within
a vessel when a radial restraint is removed, while the former
relies on an externally-applied force to configure it from a
crimped or stowed state to the deployed or expanded state.
Self-expanding stents are designed to expand significantly when a
radial restraint is removed such that a balloon is often not needed
to deploy the stent. Self-expanding stents do not undergo, or
undergo relatively no plastic or inelastic deformation when stowed
in a sheath or expanded within a lumen (with or without an
assisting balloon). Balloon expanded stents or scaffolds, by
contrast, undergo a significant plastic or inelastic deformation
when both crimped and later deployed by a balloon.
In the case of a balloon expandable stent, the stent is mounted
about a balloon portion of a balloon catheter. The stent is
compressed or crimped onto the balloon. Crimping may be achieved by
use of an iris or sliding-wedge types, or other types of crimping
mechanisms. A significant amount of plastic or inelastic
deformation occurs both when the balloon expandable stent or
scaffold is crimped and later deployed by a balloon. At the
treatment site within the lumen, the stent is expanded by inflating
the balloon. The expanded state is achieved and maintained,
substantially, if not entirely by an irreversible or inelastic
strain at the crowns of the stent or scaffold caused by the balloon
expansion. Self-expanding stents or scaffolds, by contrast, achieve
and maintain their expanded state in the vessel by an elastic,
radially outward force.
A film-headed crimper has been used to crimp stents to balloons.
Referring to FIG. 1A, there is shown a perspective view of a
crimping assembly 20 that includes three rolls 123, 124, 125 used
to position a clean sheet of non-stick material between the
crimping blades and the stent prior to crimping. For example, upper
roll 125 holds the sheet secured to a backing sheet. The sheet is
drawn from the backing sheet by a rotating mechanism (not shown)
within the crimper head 21. A second sheet is dispensed from the
mid roll 124. After crimping, the first and second (used) sheets
are collected by the lower roll 123. As an alternative to rollers
dispensing a non-stick sheet, a stent may be covered in a thin,
compliant protective sheath before crimping.
FIG. 1B illustrates the positioning the first sheet 125a and second
sheet 124a relative to the wedges 22 and a stent 100 within the
aperture of the crimping assembly 20. As illustrated each of the
two sheets are passed between two blades 22 on opposite sides of
the stent 100 and a tension T1 and T2 applied to gather up excess
sheet material as the iris of the crimping assembly is reduced in
size via the converging blades 22.
The dispensed sheets of non-stick material (or protective sheath)
are used to avoid buildup of coating material on the crimper blades
for stents coated with a therapeutic agent. The sheets 125a, 124a
are replaced by a new sheet after each crimping sequence. By
advancing a clean sheet after each crimp, accumulation of
contaminating coating material from previously crimped stents is
avoided. By using replaceable sheets, stents having different drug
coatings can be crimped using the same crimping assembly without
risk of contamination or buildup of coating material from prior
stent crimping.
There is a continuing need to improve upon methods for crimping a
medical device and, in particular, a polymer scaffold to a delivery
balloon in order to improve upon the uniformity of deployment of a
polymer scaffold from the balloon, to increase the retention force
between scaffold and balloon, and to obtain a minimal crossing
profile for delivery of the scaffold to a target site.
SUMMARY OF THE INVENTION
The invention provides methods for crimping a balloon-expanded
scaffold to a balloon catheter. According to one embodiment the
inventive methods disclosed herein are used to improve upon a
crimping process for a thin-walled scaffold. The process may
alternatively be used to improve-upon a crimp process used to crimp
scaffolds that have thicker walls.
Referring to the case of a thin-walled scaffold, it has been
realized through testing a need to modify aspects of a crimping
process that did not pose significant problems when a higher wall
thickness scaffold was crimped using the same process. An example
of a scaffold having a higher wall thickness is described in US
2010/0004735. It has been found that when a significant reduction
in wall thickness is made (e.g., from 158 microns or about 160
microns wall thickness down to 100 microns wall thickness or less)
prior methods of crimping have proven unsatisfactory. Those prior
methods of crimping produced high numbers of twisted, cracked or
fractured struts when applied to thin-walled scaffolds.
According to the invention, it has been determined that
modifications to a crimping process may better ensure that all four
of the following objectives are met: Structural integrity: avoiding
damage to the scaffold's structural integrity when the scaffold is
crimped to the balloon, or expanded by the balloon. Safe delivery
to an implant site: avoiding dislodgement or separation of the
scaffold from the balloon during transit to an implant site and
having a small crossing profile for the catheter. Uniformity of
expansion: avoiding non-uniform expansion of scaffold rings, which
can lead to structural failure and/or reduced fatigue life.
Avoidance of balloon over-stretch: monitoring of balloon pressure
in relation to decreasing scaffold size to avoid excessive strain
or possible pin-hole leaks in the balloon and without compromising
the three prior needs.
According to the embodiments, a polymer scaffold is crimped to a
balloon of a balloon catheter using a crimping device and polymer
material disposed between the surfaces of the scaffold and faces of
crimper blades that bear down on the scaffold during crimping. In a
preferred embodiment the polymer material are sheets provided with
a film-headed crimping device. According to this embodiment, the
scaffold is crimped down in intermittent fashion. Between one or
more crimping stages the polymer sheets are adjusted to remove
slack or excess accumulated sheet material. After this re-setting
of the polymer sheets the scaffold diameter is reduced down
further, which may be followed subsequently by another re-setting
of the polymer sheets, as necessary or desired. The number of
re-sets of the polymer sheets will in general depend on the degree
of diameter reduction during crimping, and more specifically will
depend upon the crimping results, type of scaffold being crimped
and material of the scaffold.
In an alternative, but less preferred embodiment the polymer
material are sheaths placed over the scaffold. According to this
embodiment a sheath having a first size is placed over the
scaffold. The scaffold diameter is then reduced down by a crimping
device. After the scaffold is partially reduced in diameter, the
first sheath is replaced by a second, smaller sheath, matching the
reduced diameter of the scaffold. The first sheath is replaced by
the second, smaller sheath to avoid interference with the crimping
process. Although using sheaths is a possible alternative way of
protecting the scaffold and avoiding interference with the movement
of struts as they fold around crowns during crimping, it is
believed a very cumbersome and time/labor intensive manner of
protecting a scaffold.
The crimping process may be used for a polymer scaffold or metal
stent. In either case the benefits of having polymer material
removed to minimize interference with the crimping process may be
necessary in order to avoid irregular crimping or damage to coating
material.
According to the various aspects of the invention, there is a
medical device, method for crimping, or method for assembly of a
medical device comprising such a medical device having one or more,
or any combination of the following things (1) through (17): (1)
The medical device is a stent or scaffold crimped to a balloon
catheter. (2) A crimping method applied using a polymer material
disclosed within a crimp aperture and between crimper blades and a
scaffold. (3) Re-setting of a polymer material within an aperture
of a crimper head. (4) A sliding wedge or iris-type crimper is used
including but not limited to a film-headed crimper. (5) The
scaffold has a before crimp diameter that is higher than a nominal
diameter for the balloon of the balloon catheter to which the
scaffold is crimped. (6) There is at least 2, between 2 and 5
re-sets of polymer material during a crimp process. (7) There is a
dwell period of between 1 and 25 seconds for a stage of a crimping
process prior to a final dwell. (8) A process for crimping a
thin-walled scaffold having a wall thickness of less than 125
microns, or less than 100 microns, or between 80 and 125 microns to
a balloon. (9) A scaffold having a pattern according to FIG. 5.
(10) Balloon pressurization during crimping may be nominal balloon
pressure, and balloon pressure decreased (or relieved) after
50%-75% of the final crimp dwell period is complete. (11) Balloon
pressure relieved after about 50% to 60% reduction from the before
crimping diameter. (12) A re-setting of the polymer material takes
place according to any combination of the following: (a) First
re-set takes place after about 30-35% reduction from the before
crimp diameter, depending on scaffold initial diameter size
(smaller starting size means re-set more likely needed in this
range). This re-set may correspond to the time when the scaffold is
removed from the crimper and alignment checked (or switching to
Balloon A); (b) Two or more re-sets may be chosen based on the
total travel from initial diameter to final crimp diameter; e.g.,
for diameter reductions of 2:1 (initial diameter to final diameter)
use 2 re-sets, for 3:1 or above 3:1 use 3 or more re-sets; (c) For
scaffold designs where struts closer together use more resets; (d)
Employ a re-set whenever there has been a diameter reduction of
about 30-35% between stages, but not to exceed in total 2, 3 or 4
re-sets for the entire crimping process; and/or (e) Limit to
maximum of 5 or between 2 and 5 re-sets. However, more re-sets are
certainly possible and may be needed to achieve a desired outcome.
(13) A method, comprising: using a scaffold made from a tube
comprising a polymer, the polymer having a glass transition
temperature, the scaffold having an outer diameter and the outer
diameter having a before crimping size; using a balloon having a
nominal diameter; using a crimping device having a plurality of
blades configured to form an aperture, wherein the blades are
rotated relative to each other to increase or decrease the size of
the aperture during crimping; using a polymer material disposed
within the aperture; and crimping the scaffold to the balloon, the
crimping comprising: placing the scaffold and balloon within the
aperture, wherein the polymer material is between a surface of the
scaffold and a surface of the blades, reducing the diameter of the
scaffold from the before crimping size to a first size, while the
scaffold has about the first size, resetting the polymer material
within the aperture, reducing the diameter of the scaffold from the
first size to a second size, while the scaffold has about the
second size, resetting the polymer material within the aperture.
(14) The method of (13), (15) or (16) in combination with one or
more, or any of items (a)-(l): (a) wherein the crimping device is a
film-headed crimper. (b) wherein the polymer material are polymer
sheets. (c) wherein the polymer material comprises a plurality of
sheaths. (d) wherein the scaffold has a crimping temperature during
crimping. (e) wherein the before crimping size is greater than the
nominal diameter of the balloon. (f) wherein the balloon is
pressurized during each of the reducing the diameter steps. (g) the
crimping step further including the step of removing the scaffold
and balloon from the crimping device after the scaffold diameter is
reduced to the first diameter, then returning the scaffold to the
crimping device. (h) wherein the re-setting of the polymer material
while the scaffold has about the first size occurs when the
scaffold and balloon are removed from the crimping device. (i)
wherein the balloon is a first balloon, further including the step
of replacing the first balloon with a second balloon of a balloon
catheter when the scaffold is removed from the crimping device, and
the scaffold is crimped to the second balloon. (j) wherein the
scaffold diameter is reduced from the before crimping diameter to
the first diameter using a first crimping device, and the scaffold
diameter is reduced from the first size to the second size using a
second crimping device. (k) wherein the polymer material within the
aperture is re-set more than 2 times during the crimping. (l)
wherein before and after reducing the scaffold diameter from the
first size to the second size the aperture is held constant while
the balloon has the nominal diameter. (15) A method, comprising:
using a scaffold made from a tube comprising a polymer, the polymer
having a glass transition temperature, the scaffold having an outer
diameter and the outer diameter having a before crimping size;
using a balloon having a nominal diameter; using a polymer material
disposable within the aperture; and using a crimping device having
a plurality of blades configured to form an aperture, wherein the
blades are rotated relative to each other to increase or decrease a
size of the aperture during crimping; and crimping the scaffold to
the balloon, the crimping comprising: placing the scaffold and
balloon within the aperture, reducing the diameter of the scaffold
from the before crimping size to a first size that is between 30%
to 35% less than the before crimping size, after reducing the
diameter to the first size, increasing the aperture size to remove
a pressure of the blades from a surface of the scaffold, followed
by removing excess polymer material from the aperture, and after
removing the polymer material, decreasing the aperture size,
reducing the scaffold diameter from the first size to a second
size, and after reducing the diameter to the second size,
increasing the aperture size to remove a pressure of the blades
from a surface of the scaffold, followed by removing excess polymer
material from the aperture. (16) A method, comprising: using a
scaffold made from a tube comprising a polymer, the polymer having
a glass transition temperature, the scaffold having an outer
diameter and the outer diameter having a before crimping size;
using a balloon having a nominal diameter; using a crimping device
having a plurality of blades configured to form an aperture; using
a polymer material disposable within the aperture; and crimping the
scaffold to the balloon, the crimping comprising: placing the
scaffold and balloon within the aperture so that the polymer
material is between a scaffold surface and a surface of the blades,
reducing the diameter of the scaffold from the before crimping size
to a second size, wherein the polymer material within the aperture
is reset between 2 and 5 times while the scaffold diameter is
reduced from the before crimping size to the second size. (17) The
method of (13), (15) or (16) in combination with one or more, or
any of items (a)-(c): (a) wherein the polymer material comprises
sheaths having different sizes. (b) wherein the polymer material
are sheets operated by a film-headed crimper. (c) wherein the
scaffold comprises struts forming rings, wherein neighboring rings
are connected to each other by at least two links, and the scaffold
is crimped to a theoretical minimum crimp size (D-min).
INCORPORATION BY REFERENCE
All publications and patent applications mentioned in the present
specification are herein incorporated by reference to the same
extent as if each individual publication or patent application was
specifically and individually indicated to be incorporated by
reference. To the extent there are any inconsistent usages of words
and/or phrases between an incorporated publication or patent and
the present specification, these words and/or phrases will have a
meaning that is consistent with the manner in which they are used
in the present specification.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a perspective view of a prior art film-headed
crimper.
FIG. 1B is a frontal view of the head of the film-headed crimper of
FIG. 1A as crimper jaws are being brought down on a stent.
FIGS. 2A-2B are scanning electron microscope (SEM) images of a
cross-section of a scaffold partially crimped to a catheter balloon
within a crimp head. Polymer sheets of the crimping mechanism are
wrapped around the scaffold with portions lodged between scaffold
struts.
FIGS. 3A and 3B describe a first process (Process I) for crimping a
scaffold according to the disclosure.
FIGS. 4A and 4B describe a second process (Process II) for crimping
a scaffold according to the disclosure.
FIG. 5 shows distal and proximal end portions of a scaffold
according to one embodiment.
FIG. 6 shows the scaffold of FIG. 5 crimped to a balloon of a
balloon catheter.
DETAILED DESCRIPTION
In the description like reference numbers appearing in the drawings
and description designate corresponding or like elements among the
different views.
Definitions
For purposes of this disclosure, the following terms and
definitions apply:
The terms "about," "approximately," "generally," or "substantially"
mean 30%, 20%, 15%, 10%, 5%, 4%, 3%, 2%, 1.5%, 1%, between 1-2%,
1-3%, 1-5%, or 0.5%-5% less or more than, less than, or more than a
stated value, a range or each endpoint of a stated range, or a
one-sigma, two-sigma, three-sigma variation from a stated mean or
expected value (Gaussian distribution). For example, d1 about d2
means d1 is 30%, 20%, 15%, 10%, 5%, 4%, 3%, 2%, 1.5%, 1%, 0% or
between 1-2%, 1-3%, 1-5%, or 0.5%-5% different from d2. If d1 is a
mean value, then d2 is about d1 means d2 is within a one-sigma,
two-sigma, or three-sigma variance or standard deviation from
d1.
It is understood that any numerical value, range, or either range
endpoint (including, e.g., "approximately none", "about none",
"about all", etc.) preceded by the word "about," "approximately,"
"generally," or "substantially" in this disclosure also describes
or discloses the same numerical value, range, or either range
endpoint not preceded by the word "about," "approximately,"
"generally," or "substantially."
The "glass transition temperature," TG, is the temperature at which
the amorphous domains of a polymer change from a brittle vitreous
state to a solid deformable or ductile state at atmospheric
pressure. This application defines TG and methods to find TG, or
TG-low (the lower end of a TG range) for a polymer in the same way
as in U.S. application Ser. No. 14/857,635.
A "stent" means a permanent, durable or non-degrading structure,
usually comprised of a non-degrading metal or metal alloy
structure, generally speaking, while a "scaffold" means a temporary
structure comprising a bioresorbable or biodegradable polymer,
metal, alloy or combination thereof and capable of radially
supporting a vessel for a limited period of time, e.g., 3, 6 or 12
months following implantation. It is understood, however, that the
art sometimes uses the term "stent" when referring to either type
of structure.
"Inflated diameter" or "expanded diameter" refers to the inner
diameter or the outer diameter the scaffold attains when its
supporting balloon is inflated to expand the scaffold from its
crimped configuration to implant the scaffold within a vessel. The
inflated diameter may refer to a post-dilation balloon diameter
which is beyond the nominal diameter, or nominal inflated diameter
for the balloon (e.g., a 6.5 mm balloon has a nominal diameter of
6.5 mm or when inflated to its nominal inflated diameter has a
diameter of 6.5 mm). The scaffold diameter, after attaining its
inflated or expanded diameter by balloon pressure, will to some
degree decrease in diameter due to recoil effects related primarily
to, any or all of, the manner in which the scaffold was fabricated
and processed, the scaffold material and the scaffold design. When
reference is made to a fully inflated diameter of a balloon, it
refers to balloon pressurization corresponding to the nominal
inflated diameter or greater than the nominal inflated
diameter.
When reference is made to a diameter it shall mean the inner
diameter or the outer diameter, unless stated or implied otherwise
given the context of the description.
"Post-dilation diameter" (PDD) of a scaffold refers to the inner
diameter of the scaffold after being increased to its expanded
diameter and the balloon removed from the patient's vasculature.
The PDD accounts for the effects of recoil. For example, an acute
PDD refers to the scaffold diameter that accounts for an acute
recoil in the scaffold.
A "before-crimp diameter" means an outer diameter (OD) of a tube
from which the scaffold was made (e.g., the scaffold is cut from a
dip coated, injection molded, extruded, radially expanded, die
drawn, and/or annealed tube) or the scaffold before it is crimped
to a balloon. Similarly, a "crimped diameter" means the OD of the
scaffold when crimped to a balloon. The "before-crimp diameter" can
be about 2 to 2.5, 2 to 2.3, 2.3, 2, 2.5, 3.0 times greater than
the crimped diameter and about 0.9, 1.0, 1.1, 1.3 and about 1-1.5
times higher than an expanded diameter, the nominal balloon
diameter, or post-dilation diameter. Crimping, for purposes of this
disclosure, means a diameter reduction of a scaffold characterized
by a significant plastic deformation, i.e., more than 10%, or more
than 50% of the diameter reduction is attributed to plastic
deformation, such as at a crown in the case of a stent or scaffold
that has an undulating ring pattern, e.g., FIG. 1. When the
scaffold is deployed or expanded by the balloon, the inflated
balloon plastically deforms the scaffold from its crimped diameter.
Methods for crimping scaffolds made according to the disclosure are
described in US20130255853.
A "crimping stage" or "stage" of a crimping process refers to a
period of time when the jaws of a crimping device are held fixed,
or the aperture of the crimp head is held at a constant diameter.
The duration of the stage may be called a dwell period. Dwell
periods can range from 1 sec to 25 sec, for initial stages prior to
a final dwell. After the final crimped diameter is reached the
dwell may be between 50 sec and 300 sec. The aperture of a crimping
device is reduced from a first diameter to a second diameter when
the crimping device moves from a first stage to a second stage,
respectively. The aperture reduction sizes--e.g., from a first
diameter or aperture size to second diameter or aperture size--are,
for purposes of this disclosure, understood as being the same as
the actual outer diameter of the scaffold within the aperture when
the scaffold is being reduced in size by the crimper crimp. It is
understood, however, that a programmed aperture size may not be
exactly the same as the outer diameter of the crimped scaffold
size, especially when a scaffold is being crimped to very small
diameters.
A material "comprising" or "comprises" poly(L-lactide) or PLLA
includes, but is not limited to, a PLLA polymer, a blend or mixture
including PLLA and another polymer, and a copolymer of PLLA and
another polymer. Thus, a strut comprising PLLA means the strut may
be made from a material including any of a PLLA polymer, a blend or
mixture including PLLA and another polymer, and a copolymer of PLLA
and another polymer.
When reference is made to a direction perpendicular to, or parallel
with/to axis A-A (e.g., as shown in FIG. 5) it will mean
perpendicular to, or parallel with/to the axial direction of a
scaffold or tube. Similarly, When reference is made to a direction
perpendicular to, or parallel with/to axis B-B (e.g., as shown in
FIG. 5) it will mean perpendicular to, or parallel with/to the
circumferential direction of the scaffold or tube. Thus, a
sinusoidal ring of a scaffold extends parallel with/to (in periodic
fashion) the circumferential direction or parallel to axis B-B, and
perpendicular to axis A-A whereas a link in one embodiment extends
parallel to the axial direction or axis A-A of the scaffold or tube
and perpendicular to the axis B-B.
Wherever the same element numbering is used for more than one
drawing it is understood the same description first used for the
element in a first drawing applies to embodiments described in
later drawings, unless noted otherwise.
The dimension of thickness (e.g., wall, strut, ring or link
thickness) refers to a dimension measured perpendicular to both of
axes A-A and B-B. The dimension of width is measured in the plane
of axes A-A and B-B; more specifically, the width is the
cross-sectional width from one side to another side of a contiguous
structure; thus, link 334 has a constant link width. Moreover, it
is understood that the so-called plane of axes A-A and B-B is
technically not a plane since it describes surfaces of a tubular
structure having central lumen axis parallel with axis A-A. Axis
B-B therefore may alternatively be thought of as the angular
component if the scaffold locations were being described using a
cylindrical coordinate system (i.e., axis A-A is Z axis and
location of a luminal/abluminal surface of a crown, link, ring,
etc. is found by the angular coordinate and radial coordinate
constant).
A "thin wall thickness," "thin-walled scaffold," "thin-wall" refers
to a strut, ring, link, or bar arm made from a polymer comprising
poly(L-lactide) and having a wall thickness less than 125
microns.
A "crimping temperature" according to the disclosure means a
temperature above ambient and slightly less than, or about equal to
the glass transition temperature (TG) for a polymer of the
scaffold, e.g., poly(L-lactide). In a preferred embodiment the
crimping temperature is between TG and 15 degrees less than TG, or
between TG and 10 degrees, or 5 degrees less than TG. In other
embodiments the crimping temperature is achieved by heating the
scaffold to a temperature at least 20 degrees below TG and
preferably to a temperature at least 15 degrees below TG.
"Re-set of the polymer material within the aperture" as indicated
in the crimping steps in FIGS. 3B and 4B or "resetting of the
polymer material within the aperture", means one or both of
removing excessive polymer material from within an aperture of a
crimp head formed by the interconnected blades or wedges of a
mechanical crimping device (e.g., an iris or sliding wedge type
crimper) or increasing/opening the aperture sufficiently to remove
blade pressure on the scaffold (in the case of a film-headed
crimper). The blades or wedges converge upon the scaffold in order
to reduce the diameter of the scaffold (and crimp the scaffold to
the balloon). As example of a film-headed crimper is the MSI.TM.
SC775S/875S, available from the Machine Solutions company. For this
crimper re-set of the polymer material within the aperture is
accomplished by fully opening the crimp aperture to cause the
polymer sheet material to automatically return to its starting
position and become fully taut and a fresh sheet of polymer
material to spool. After this step, the aperture is then brought
back down upon the scaffold to continue the crimping process.
Embodiments
An effective crimping process for a scaffold must at least satisfy
each of the following objectives: Structural integrity: avoiding
damage to the scaffold's structural integrity when the scaffold is
crimped to the balloon, or expanded by the balloon. Safe delivery
to an implant site: avoiding dislodgement or separation of the
scaffold from the balloon during transit to an implant site.
Uniformity of expansion: avoiding non-uniform expansion of scaffold
rings, which can lead to structural failure and/or reduced fatigue
life.
As previously reported in US20140096357 a scaffold is not as
resilient as a stent made from metal, which is highly ductile.
Satisfying all of the above needs is therefore more challenging for
a polymer scaffold, especially a thin-walled scaffold that can
fracture more easily during crimping or balloon expansion and is
more susceptible to twisting, flipping or overlap during
crimping.
According to the disclosure there is a crimping process that
includes steps where polymer material is re-set or replaced in the
crimp head in order to minimize any interference between the
compressing-down of the scaffold struts by crimper blades and the
polymer material. The polymer material is used to protect the
surface or the scaffold, or coating disposed over a scaffold (or
stent). However, as the scaffold is crimped further down and its
diameter decreases, the polymer material surrounding the scaffold
when it had the larger diameter becomes excessive, resulting in
folds, roll-up, slackening or loss of tension. Although a crimping
mechanism may include a tensioning portion that applies a
tensioning force as the aperture decreases (as a means to take-up
excess slack in the polymer material) due to the presence of the
blades in close proximity, or in contact with surfaces of the
scaffold struts the tensioning force cannot remove material from
near the scaffold. To address this problem a crimp aperture is
opened and sheet material re-set (or replaced, in case of using
sheaths).
FIGS. 2A-2B illustrates what happens when polymer sheet material
becomes slack when the diameter and blades are not removed to
re-set the sheets, or the sheets are not otherwise kept relatively
taut near the scaffold surface. Shown is the inside of the crimp
head of a film-headed crimper. Although the film-headed crimper
includes the tensioning mechanism mentioned above, sheet material
nonetheless becomes lodged between struts of the scaffold because
the blades' proximity to the scaffold surface limits the
effectiveness of the tensioning mechanism. Basically, during a
crimp stage or diameter reduction between stages the blades are
pressing down on the scaffold surface, or the blades are very near
the scaffold surface, thereby restraining movement of the polymer
material disposed between the blades and scaffold surface when
tension is applied to the sheet material portions outside of the
aperture. The tension applied outside the blade is reacted by a
pinching force on the polymer material resulting from polymer
material being pinched between the blade and scaffold. As shown the
scaffold 300 (partially crimped to balloon 15) has struts 330.
Portions 128 of the sheets 124a/125b are caught between the folding
struts 330. As these struts attempt to fold about crowns, thereby
reducing ring sizes and diameter of the scaffold, the slack polymer
material 128 is drawn or pushed into open spaces between struts by
the converging blades. This can be easily seen in FIGS. 2A-2B.
Particularly for thin-walled scaffold struts, excessive interaction
of the pinched sheets with the folding struts tends to result in
unsatisfactory crimped units.
Re-setting or removal of the excessive polymer material after
diameter reductions (by withdrawing the blades or increasing the
aperture size, in order to allow the outside tensioning to pull the
polymer material away from the scaffold surface) was found to make
a significant difference in the quality of crimp or production
yield. It was found through testing and experimentation that a
re-set or removal of excessive polymer sheet material (or in the
alternative embodiment replacing a first sheath with a second,
smaller sheath) at critical times (as explained below), following a
diameter reduction, can prevent the polymer material from
significantly interfering with the desired folding of ring struts
about crowns in subsequent diameter reduction steps.
As discussed earlier in reference to FIG. 1B, for the film-headed
crimper a first sheet 125a and a second sheet 124a are positioned
relative to the wedges or blades 22 of the crimping device while
the scaffold (or stent 100) is within the aperture of the crimping
assembly 20. The two sheets are passed between two blades 22 on
opposite sides of the stent 100 and a tension T1 and T2 applied to
gather up excess sheet material as the iris of the crimping
assembly is reduced in size via the converging blades 22. Although
this tensioning mechanism is intended to keep the sheets relatively
taut, the sheet material nonetheless builds up in an unacceptable
manner, as explained above.
FIGS. 3A, 3B (Process I) and FIGS. 4A, 4B (Process II) are flow
diagrams illustrating two examples of crimping processes that can
achieve the foregoing objectives for scaffolds, including
thin-walled scaffolds. In each of these examples the scaffold
crimped to the balloon is laser cut from a radially expanded tube.
However, the crimping process is not limited to a scaffold made
from a laser-cut tube. Other scaffold types, e.g. a scaffold not
radially expanded, or scaffolds fabricated from a polymer sheet (as
opposed to a tube) are within the scope of disclosure.
Additionally, the starting outer diameter sizes for the scaffold,
e.g. a coronary scaffold, can be between 3.0 mm and 4.25 mm, or
between 6 mm-10 mm, outer diameter size for a peripheral
scaffold.
Crimping Processes I and II may use one or two balloons. The two
balloons referred to in the figures and below discussion are called
"Balloon A" and "Balloon B." The Balloon A refers to the balloon of
the balloon catheter of the finished product. The Balloon B refers
to a temporary or sacrificial balloon, or balloon catheter that is
used during the initial stages then replaced by the Balloon A at
the time of a final alignment check, as explained below. Practice
of the Process I or Process II using Balloon B (later replaced by
Balloon A) is desirable when the starting inner diameter size of
the scaffold is larger than, or the same size as the diameter of
the Balloon A when Balloon A is inflated to its nominal inflation
diameter, or when Balloon A is inflated beyond this size.
In a preferred embodiment of a crimping process a film-headed
crimper is used to crimp the scaffold to the balloon catheter. For
a film-headed crimper, polymer material in the form polymer sheets
dispensed from a pair of rolls (FIGS. 1A-1B) is used to protect the
scaffold from the blades of the crimper. Thus for this type of
crimper "the re-set of polymer material within the aperture" steps
means the process of opening the aperture to cause automatic
removal and re-tensioning of the polymer sheets. It will be
understood, however, that the invention is not limited to using a
film-headed crimper, and may be practiced by alternative
arrangements for placing and removing or re-setting of polymer
material within the crimp aperture, e.g., using multiple
sheaths.
Referring to FIGS. 3A-3B, two crimper settings or setups are used.
The first crimper setup is used for the crimping stages that
precede a final alignment check (FIG. 3A) and the second crimper
setup is used for the stages that follow the final alignment check
(FIG. 3B).
Pre-Crimp Procedure:
The scaffold is placed on Balloon A (or Balloon B if two balloons
will be used). The balloon is inflated to its nominal diameter or
post-dilation diameter (greater than nominal diameter size) or,
more generally, the balloon is fully inflated so that its size is
at least equal to or exceeds the inner diameter of the scaffold in
order to support the scaffold during the initial crimping steps.
The scaffold is aligned with proximal and distal markers on the
balloon (not necessary if Balloon B is used). The crimper head,
scaffold and/or balloon may also be deionized to remove static
charge buildup that can cause the scaffold to shift out of
alignment with balloon markers during crimping. Static charge
buildup has been found to not only cause misalignment between the
scaffold and balloon, but also cause irregular crimping of the
scaffold (metal stents typically do not have static charge buildup
because the balloon is in sliding contact with a metal, as opposed
to a polymer surface). The scaffold is then inserted into the
crimper head while the balloon remains fully inflated.
Stage I:
The scaffold supported on the fully inflated balloon is within the
crimp head. The temperature for crimping or crimping temperature is
set during this stage, as is the starting iris or aperture size
corresponding to the input outer diameter of the scaffold (e.g. 3.5
mm). In a preferred embodiment blades of an iris or sliding wedge
crimping device are heated to achieve the desired crimping
temperature (alternatively a heated fluid may be used). After the
scaffold reaches the crimping temperature, the iris of the crimper
closes to reduce the scaffold inner diameter (ID) to less than the
outer diameter (OD) of the fully inflated balloon and while the
balloon remains fully inflated.
Stage II:
The crimper jaws are held at a fixed diameter for a dwell period
and while the balloon is fully inflated. At the conclusion of this
dwell period the scaffold and fully inflated balloon are removed
from the crimping device.
Verify Alignment/Replace Balloon:
Removal after Stage II may be skipped if there is no need to check
or verify final alignment with balloon markers, or if Balloon A is
used for Stages I and II. In the illustrated embodiment the
scaffold supported on the fully inflated balloon is removed from
the crimping device to verify that the balloon is located between
the balloon markers (when Balloon A used for Stages I and II), or
Balloon B is replaced with Balloon A and the scaffold aligned with
the balloon markers.
Referring now to FIG. 3B, Process I continues. The crimping steps
illustrated in FIG. 3B use a crimping setup different from the
crimping setup in FIG. 3A.
Stage III:
After the scaffold and fully inflated Balloon A are returned to the
crimper, the iris diameter is set at a slightly higher diameter
than the scaffold diameter at the conclusion of Stage II (to
account for recoil). The iris or aperture size is held constant for
a time period sufficient to bring scaffold temperature back to
crimping temperature.
After the crimping temperature is reached, the scaffold diameter is
reduced down while the balloon is pressurized. The balloon is
preferably fully inflated for the diameter reduction following
Stage III.
Stage IV:
The crimp aperture is held constant for a dwell period after
scaffold diameter is reduced from the Stage III diameter. Following
Stage III the polymer sheets of the film headed crimper are re-set
to remove excess sheet material from within the aperture when the
scaffold diameter was reduced from the Stage II diameter to the
Stage IV diameter, or when the diameter was reduced from the
initial diameter to the Stage IV diameter.
Balloon pressurization in the crimping process helps ensure, or
improves scaffold retention on the balloon. The pressure is
relieved after 50%-75% of the final crimp dwell period is complete.
Typically 75-250 psi is applied. The pressure is selected to
achieve the lowest possible crossing profile and ensure sufficient
retention.
Stages V-VIII:
These stages follow a similar process as in Stages III-IV: perform
a dwell at each of the stages with a diameter reduction between the
stages. After the dwell period, the aperture is fully opened and
the excess polymer sheet material removed from the aperture. In
total there are three illustrated re-sets of the polymer material
in the example of FIGS. 3A-3B. The re-sets all occur following the
final alignment check.
Optional Stages/Final Crimp:
Following the re-set (immediately after Stage VIII) there may be a
number of additional, optional stages. At the conclusion of these
stages there is a final pressurization of the balloon at the final
crimp diameter. The pressurization may be a leak check. After this
final step the scaffold is fully crimped to the balloon catheter,
removed from the crimp head and placed within a constraining
sheath.
FIGS. 4A-4B describe an alternative crimping process. The
description accompanying FIGS. 3A-3B applies in the same manner to
FIGS. 4A-4B, except as follows. A different crimper device or setup
is used for Process I after the final alignment check. Step III
through Step VIII in Process I is performed on a different crimper
device or setup. A re-set of the polymer material therefore may be
automatically done at the time of the final alignment check in
Process I (after Stage II and before Stage III). This is why a
re-setting of polymer material within aperture is not shown in FIG.
3A. In Process II a single crimping device or setup (recipe) is
used for the crimp. At the conclusion of Stage II of Process II
(FIG. 4A) the polymer material is re-set. The re-set may be done
before or after the alignment check and/or changing of balloons
(when Balloon B is used for Stage I and Stage II), assuming the
final alignment check is even done (this step is optional in some
embodiments). Process I and Process II have a total of four
illustrated steps where polymer material within the aperture is
re-set. For Process I there may be an additional re-set step that
is essentially done when the second crimping device/setup is used
following the alignment check (thus, bring total of 4 re-sets for
Process I). The number of re-sets for a particular scaffold size,
balloon size and associated D-min (defined below) is chosen in an
optimal fashion, based on examination of the scaffolds crimped to
balloons. The criterion used to judge the effectiveness of a
selected number of re-sets was the foregoing three listed
objectives for crimping (structural integrity, scaffold retention
and uniform expansion). It will be appreciated that polymer
material interference with strut folding, especially the kind
illustrated in FIGS. 2A-2B, can negatively affect any, or all three
of the crimping objectives. Balanced against the desire to re-set
polymer material is the time needed to re-set and output yield
benefits. Decreasing the amount of diameter reduction between each
stage, followed by a re-setting of material each time may, or may
not necessarily make a big difference in crimp quality, but it
would likely make the crimp process prohibitively complicated and
time-consuming (especially for production-level crimping).
Critical Crimping Periods
According to one embodiment, a re-set of the polymer material
should be employed whenever the space between struts is large
enough to receive sheet material (near final crimp diameters spaces
between struts may be too small for sheet material) and there has
been a sufficient percentage of diameter reduction to cause
material between the blades and scaffold surface to build up. This
period of diameter reduction and resulting crimp size will be
referred to as a critical crimping period.
The number of re-sets cannot be excessive because then the crimp
process becomes too time consuming. Thus, it is not believed
feasible or cost-effective to implement a re-set whenever the
scaffold is reduced in diameter. A balance is needed. Re-set points
within critical crimping periods should be chosen so that
production yield is favorable but crimp time does not become overly
burdensome.
Based on extensive testing of different scaffold types, critical
crimp periods may employ one or more re-set of polymer material
within the aperture ("re-set") according to one or more of the
following rules: A first re-set employed after about 30-35%
reduction from the initial diameter, depending on scaffold initial
diameter size (smaller starting size means re-set more likely
needed in this range). This re-set may correspond to the time when
the scaffold is removed from the crimper and alignment checked (or
switching to Balloon A); Two or more re-sets may be chosen based on
the total travel from initial diameter to final crimp diameter;
e.g., for diameter reductions of 2:1 (initial diameter to final
diameter) use 2 re-sets, for 3:1 or above 3:1 use 3 or more
re-sets; For scaffold designs where struts closer together use more
resets; Employ a re-set whenever there has been a diameter
reduction of about 30-35% between stages, but not to exceed in
total 2, 3 or 4 re-sets for the entire crimping process; and/or
Limit to maximum of 5 or between 2 and 5 re-sets. However, more
re-sets are certainly possible and may be needed to achieve a
desired outcome.
Scaffold and Catheter
FIG. 6 illustrates a side-view of a scaffold 300 crimped to a
balloon catheter, which has a shaft 2, balloon 15 with distal and
proximal ends 17a, 17b (where balloon markers are found). The
catheter is supported on a mandrel 8.
FIG. 5 shows a partial, planer view of end portions of the scaffold
300 from FIG. 6 in an expanded or before-crimping state. This
figure illustrates an example of a network of struts and links for
the scaffold 300. The left or distal end portion 302 (i.e. the left
side of FIG. 5) includes sinusoidal rings 312a, 312b, and 312c
where ring 312a is the outermost ring. Ring 312a and ring 312b are
adjoined by two links 334 and a marker link 20. Ring 312c and ring
312d are adjoined by three links 334 that extend parallel to axis
A-A. The links 334 extend parallel to axis A-A and have a constant
cross-sectional moment of inertia across its length, meaning link
334 has a constant width and thickness and the location of the
centroid or geometric center (or longitudinal axis) of the link is
everywhere parallel with axis A-A. The right or proximal end
portion 304 (i.e. the right side of FIG. 3) includes sinusoidal
rings 312d, 312e, and 312f where ring 312f is the outermost ring.
Ring 312d and ring 312e are adjoined by three links 334. Ring 312e
and ring 312f are adjoined by two links 334 and the marker link 20.
Thus, scaffold 300 has a marker link 20 extending between and
adjoining the outermost link with the adjacent, inner ring. The
scaffold 300 may have 15, 18 or 20 rings 312 interconnected to each
other by links 334.
A ring 312, e.g., ring 312b, is sinusoidal meaning the curvature of
the ring along axis B-B is best described by a sine wave where the
wavelength of the sine wave is equal to the distance between
adjacent crests 311a of the ring. The ring has a constant width at
both crowns 307, 309 and 310 and struts 330, which connect a crown
to an adjacent crown.
There are three crown types present in each inner ring 312b through
312e: U-crown, Y-crown and W-crown. Outermost rings have only the
Y-crown or W-crown type, and the U-crown type. A crest or peak 311a
(or trough or valley 311b) may correspond to a U-crown, Y-crown or
W-crown. For the outermost ring 312a there is only a U-crown and
W-crown type. For the outermost ring 312f there is only a U-crown
and Y-crown type. A marker link 20 adjoins rings by forming a
W-crown with the first ring (e.g., ring 312e) and a Y-crown with
the second ring (e.g. ring 312f).
A link 334 connects to ring 312f at a Y-crown 310. A "Y-crown"
refers to a crown where the angle extending between a strut 330 of
a ring 312 and the link 334 is an obtuse angle (greater than 90
degrees). A link 334 connects to ring 312a at a W-crown 309. A
"W-crown" refers to a crown where the angle extending between the
strut 330 and the link 334 is an acute angle (less than 90
degrees). A U-crown 307 is a crown that does not have a link
connected to it. Marker link 20 connects to a ring at a W-crown 314
and a Y-crown 316.
For the scaffold 300 there are 6 crests or peaks 311a and 6 troughs
or valleys 311b for each ring 312. A crest 311a is always followed
by a valley 311b. Ring 312b has 12 crowns: 3 are W-crowns 309, 3
are Y-crowns 310 and 6 are U-crowns 307.
A crimped diameter enforced on scaffold 300 (using, e.g., Process I
or Process II) may be expressed in terms of a theoretical minimum
crimped diameter where struts that converge at the same crown are
in contact with each other when the scaffold is fully crimped,
i.e., when the scaffold is removed from the crimping device, or
when placed within a restraining sheath soon after crimping. The
equation for the theoretical minimum crimped diameter (D-min) under
these conditions is shown below
D-min=(1/.pi.).times.[(n.times.strut_width)+(m.times.link_width)]+2*t
Where "n" is the number of struts in a ring (12 struts for scaffold
300), "strut_width" is the width of a strut (170 microns for
scaffold 300), "m" is the number of links adjoining adjacent rings
(3 for scaffold 300), "link_width" is the width of a link (127
microns for scaffold 300), and "t" is the wall thickness (93
microns for scaffold 300).
Hence, for scaffold 300
D-min=(1/.pi.).times.[(12.times.170)+(3.times.127)]+2.times.(93)=957
microns. As can be appreciated D-min according some embodiments for
crimping is not a function of a non-zero inner crown radius (as
will be appreciated if the crimping did not exceed the inner crown
radius then this additional sum of distances, i.e., twice the inner
crown radius for each crown of a ring, would be added to D-min).
Thus D-min defined above is less than a D-min where crimping does
not bring struts into contact.
The above description of illustrated embodiments of the invention,
including what is described in the Abstract, is not intended to be
exhaustive or to limit the invention to the precise forms
disclosed. While specific embodiments of, and examples for, the
invention are described herein for illustrative purposes, various
modifications are possible within the scope of the invention, as
those skilled in the relevant art will recognize.
These modifications can be made to the invention in light of the
above detailed description. The terms used in claims should not be
construed to limit the invention to the specific embodiments
disclosed in the specification.
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